The components used in optical networks are often complex structures, individually fabricated for specific applications of use. Though complex overall, many of these components are formed of relatively simple individual optical devices combined to achieve complex functionality. Just as the advent of semiconductor logic gates facilitated the creation of the microprocessor, the development of simple optical devices performing functions such as coupling, splitting, and constructive/destructive interference allows system designers to form increasingly complex optical circuits.
Of the various basic optical structures, signal splitting is one of the most important. Generally, signal splitting is achieved through either direct or indirect coupling techniques. Indirect coupling, for example, relies upon evanescent field coupling through two close proximity waveguides, one being a source waveguide. Direct coupling instead involves bringing an input waveguide (or propagating medium) in direct physical contact with one or more output waveguides. Y-branches and multimode interference (“MMI”) couplers are two examples of direct coupling structures that can be used to split an optical signal.
Y-branches are the most common direct coupling structures for implementing an optical splitter. FIG. 1 is a block diagram illustrating a known Y-branch 100 for splitting an input optical signal 105 into two output optical signals 110A and 110B. Y-branch 100 includes an input section 115 (for receiving input optical signal 105) coupled to two branching sections 120A and 120B. Where branching sections 120A and 120B meet, a sharp inner edge, called a splitting point 125, is defined having a splitting angle φ greater than zero (typically much greater than zero). Branching sections 120A and 120B diverge from splitting point 125 with a radius of curvature R1.
Y-branch 100 loses a sizeable amount of input energy due to a mode mismatch at the splitting point 125, which causes back reflections and radiation seepage and further due to limitations in device fabrication. Fabrication of Y-branch 100 is a lithographic process in which high-quality lithography equipment, such as E-beam lithography equipment is used. Even with such equipment, it is difficult to fabricate well-aligned and symmetric branching sections 120A and 120B defining a sharp and centered splitting point 125. These difficulties are compounded as optical devices continued to shrink in size. Even if perfect alignment of branching sections 120A and 120B and a well defined splitting point 125 were to be achieved in one device, reproducing such alignment and well defined feature across a batch of fabricated devices is not likely.
To avoid the high cost associated with high-quality lithography equipment, lower quality lithography techniques are generally used. Of course, there is a tradeoff between cost and quality. A poor quality inner edge at splitting point 125 results in power loss due to light spill out between branching sections 120A and 120B (see FIG. 5) and non-uniform split power ratios. For example, each branching section of a 50/50 Y-branch splitter may receive much less than the ideal 50% of the optical input power, and further, the optical input power that is coupled to each of the branching sections typically varies between the branching sections by 30%. These imperfections are compounded in applications such as a multi-fanout “H-Tree” where successive levels of Y-branches are coupled together. For example, where an optical power split non-uniformity of X% occurs on average due to fabrication imperfections, an optical device having N levels of Y-branches can result in N·X% non-uniformity after N levels of Y-branches. Thus, current fabrication imperfections can render entire optical devices inoperable.